1. Technical Field
This invention applies to gas turbine rotor blades in general, and to cooled gas turbine rotor blades in particular.
2. Background Information
Turbine sections within an axial flow turbine engine include rotor assemblies that include a disc and a number of rotor blades. The disk includes a plurality of recesses circumferentially disposed around the disk for receiving the blades. Each blade includes a root, a hollow airfoil, and a platform. The root includes conduits through which cooling air may enter the blade and pass through into a cavity within the hollow airfoil. The blade roots and recesses are shaped (e.g., a fir tree configuration) to mate with one another to retain the blades to the disk. The mating geometries create a predetermined gap between the base of each recess and the base of the blade root. The gap enables cooling air to enter the recess and pass into the blade root.
Airflow pressure differences propel cooling air into and out of the rotor blade. Relatively high pressure cooling air is typically bled off of a compressor section. The energy imparted to that air enables the requisite cooling, but does so at a cost since that energy is no longer available to create thrust within the engine. Hence, it is desirable to minimize the amount of energy that is necessary to provide cooling within a rotor blade.
The gas path pressure external to a rotor blade airfoil is highest at the leading edge region during operation of the blade. In many turbine applications, airfoils are typically backflow margin limited at the leading edge of the airfoil. The term “backflow margin” refers to the ratio of internal pressure to external pressure. To ensure hot gases from the external gas path do not flow into an airfoil, it is necessary to maintain a particular predetermined backflow margin that accounts for expected internal and external pressure variations. Hence, it is desirable to minimize pressure drops within the airfoil to the extent possible, particularly with respect to passages providing airflow to cool the leading edge.
It is known to use conduits within a blade root having a bellmouth inlet; i.e., an inlet that is flared on the leading edge (“forward”) side, suction side, pressure side, and the trailing edge (“aft”) side. A disadvantage of this approach is that the bellmouth inlet decreases the size of the root material that extends between the suction side and pressure side, between adjacent conduits. During operation, the blade root is highly loaded between the suction and pressure sides. Decreasing the cross-sectional area of root material between the suction and pressure sides undesirably decreases the ability of the root to handle the load.
What is needed is a rotor blade that requires less energy to be adequately cooled relative to prior art rotor blades, one that requires less energy for cooling by reducing pressure losses within the rotor blade relative to prior art rotor blades, and one that can adequately handle the attachment loading within the root.